This invention relates generally to surgically implanted delivery and drainage catheters, such as in shunt systems that drain cerebrospinal fluid from the brain ventricles and drug delivery catheters implanted in fluid filled spaces or within the parenchyma of tissues. More particularly, this invention is an improved catheter that prevents the ingrowth of tissue and subsequent blockage of such catheter.
As is well known in the medical arts, to relieve undesirable accumulation of fluids it is frequently necessary to provide a means for draining a fluid from one part of the human body to another in a controlled manner. This is required, for example, in the treatment of hydrocephalus, an ailment usually afflicting infants or children in whom fluids which ought to drain away instead accumulate within the brain and thereby exert extreme pressure and skull deforming forces.
In treating hydrocephalus, cerebrospinal fluid accumulated in the brain ventricles is drained away by a catheter inserted into a ventricle through the skull, and the catheter is connected to a tube which conducts the fluid away from the brain to be reintroduced into the vascular system, as by extending through the patient's jugular vein to the atrium portion of the heart or to the peritoneul cavity of the abdomen. To control the flow of cerebrospinal fluid and maintain the proper pressure in the brain ventricle, a valve is generally placed in the conduit between the brain and the heart. The brain ventricles are normally large enough to easily accommodate an end of a catheter several millimeters in diameter. Such ventricular catheters are commonly provided with numerous small holes approximately 0.25-0.50 millimeters (250-500 micrometers) in diameter through their walls for receiving cerebrospinal fluid from the ventricle. To insert the ventricular catheter, a hole is bored through the skull and a solid stylet (such as that shown in U.S. Pat. No. 5,098,411) is utilized as an introducer to properly position the flexible catheter within the brain ventricle. Since the openings in the wall of the catheter are of substantial size, tissue can easily infiltrate them over time. Operative revisions to replace occluded ventricular catheters are quite common and are the leading cause of hydrocephalus shunt revisions.
Additionally, catheters are often placed within the ventricles of the brain, other fluid filled body cavities and/or directly within the tissues of the target organ for the purpose of targeted delivery of therapeutic substances. These catheters can also be infiltrated with tissue that compromises the flow of the drug being delivered.
In 1992, the inventor co-authored a paper (Neurosurgery, Vol. 31, No. 6, December 1992) on an attempted application of expanded polytetrafluoroethlene (e-PTFE) for the purposes of producing a catheter for use in hydrocephalus drainage that would be more resistive to bacterial colonization than the conventional silicone catheter. This paper described the failure of experimentation with a micro-porous e-PTFE catheter. E-PTFE was selected as a candidate material not for its porosity, but for its surface chemistry. E-PTFE was known to inhibit bacteria colonization, but was too stiff for a catheter material. For those prototype catheters, the e-PTFE was expanded to the minimum porosity that was technologically possible at the time, approximately 5 micrometers. Expanding the material retained its surface chemistry but greatly improved its flexibility. The expansion process of the e-PTFE yields a supple material like silicone whereas the unexpanded PTFE is too generally stiff for long-term implantation. As demonstrated in the paper, the catheter segments that were implanted with e-PTFE of 5 and 30 micrometers internodal distances occluded rapidly with tissue ingrowth.
Thus, catheters having a porosity of 5 micrometers or greater are unsuitable for indwelling catheters as they become occluded due to tissue ingrowth. It is important to emphasize that this paper was studying material properties that resisted bacterial colonization and the adverse reaction of tissue infiltration directly into the structure of the polymer was a finding that excluded porous materials for further consideration for that application. Because of this clinical failure, this paper taught away from the use of porous materials for hydrocephalus drainage applications.
Micro-porous membranes are commonly utilized in the field of cell encapsulation. In this application, the membrane is used to provide a means of isolating living cells within a closed capsule from the host immune system. The membrane in this application is permeable to body fluids, proteins, glucose, and the by-products of the encapsulated cells, yet impermeable to the host cells and large immune system molecules. Similar membranes are extensively used in the field of dialysis. In this application, a patient's blood is passed through the inner lumen of the membrane and only the cells or molecules of selected sizes are allowed to pass though the membrane and out of the patient's blood. The manufacture and control of the porosity of micro-porous hollow fiber membranes for use in such applications are suitably defined in the prior art, such as U.S. Pat. No. 5,284,761, which is incorporated by reference herein. In these applications, the porosity of the membrane is extremely small, yielding membranes unsuitable for the mass flow of fluids necessary to provide adequate drainage for hydrocephalus applications, wound drainage or drug delivery. Additionally, these membranes are extremely fragile with break forces generally less than 0.5 Newton (50 grams) making them unsuitable for conventional introduction into the body, as with indwelling catheters.
Accordingly, there has been a continuing need for an improved indwelling catheter that can provide continuous flow of fluids without allowing tissue infiltration. Such a catheter should be capable of being introduced conventionally, and it should be of simplified construction utilizing materials which are easily sterilizable and compatible for biomedical usage. The present invention fulfills these needs and provides other related advantages.